Turbochargers for use with internal combustion engines are known. A typical turbocharger includes a turbine that is connected to a compressor through a center-housing. During operation, exhaust gas from the engine passes through the turbine and causes a turbine wheel to rotate. The rotating turbine wheel is connected to an end of a shaft that extends through the center-housing into the compressor. A compressor wheel connected to an opposite end of the shaft rotates and, thus, operates to compress air entering the engine. Operation and efficiency of the compressor, in general terms, are limited by the size of the compressor, as well as by the diameter of an inlet opening to the compressor, which is also known as an inducer opening or diameter. Under certain operating conditions, for example, when the compressor operates close to a surge condition, it is possible to improve the efficiency of the compressor by introducing a recirculation passage.
In a typical compressor, the recirculation passage is an annular volume or cavity that surrounds the inducer opening. The recirculation volume is open on both ends to an inlet bore of the compressor, and serves to recirculate at least some air from a region around the trailing edges of the compressor wheel blades, to a region upstream of the compressor wheel but still within the inlet bore of the compressor. The recirculation passage can be defined between an inner surface of the inlet bore of the compressor, and an outer surface of an inlet collar, the inlet collar defining internally the inducer bore. The inlet collar is typically cast unitarily with the compressor housing, and is connected to the housing by a plurality of posts.
It has been found that placement of the posts within the recirculation volume can, under certain operating conditions, affect the performance of the turbocharger. For instance, it has been found that a symmetrical orientation of three posts within the recirculation volume generates regions of fluctuating pressure in areas adjacent to the trailing edges of the compressor wheel blades. These pressure fluctuations can expose the compressor wheel blades to fluctuating stresses that, under resonance conditions, have been known to cause cracks and even structural failures in the tips of compressor wheel blades. Moreover, the fluctuating pressures in the recirculation passage can cause unwanted audible noise.
It has been found that asymmetrical placement of the posts within the recirculation passage can resolve issues caused by the resonance of fluctuating pressures. It is believed that the asymmetrical placement of the posts can disrupt standing waves that cause the pressure fluctuations and avoid the creation of fluctuating stresses to the compressor wheel blades, as well as reduce or eliminate the noise that is created. Hence, newer designs for compressor housings having asymmetrical post arrangements have been used on new turbochargers.
Compressor housings are typically formed in a casting operation, and finished with a series of machining operations. The machining operations form the various features of the compressor housing, and can include operations that grind and/or cut the various features out of a “raw” casting. While these machining operations are being performed, special care is taken when forming surfaces or features of the compressor housing that will interact with other components. For example, when forming a portion of the inducer diameter in an inlet collar of the compressor, care is taken to maintain a proper diameter opening and proper position of the inducer diameter because of its proximity to the rotating blades of the compressor wheel when the compressor is fully assembled. Formation of the inducer opening is often accomplished by setting the compressor housing into a fixture that locates the position of the inducer opening with respect to machining targets or datum targets that are formed in the casting as reference points or areas.
When a used turbocharger unit having a symmetrical inlet post configuration (e.g., a three-post symmetrical configuration) is returned to a re-manufacturer for rebuilding, reconditioning, or updating, the used compressor housing having the symmetrical post configuration may be replaced with a new compressor housing having a non-symmetrical inlet post configuration (e.g., a four-post non-symmetrical configuration). Even though replacement of compressor housings on returned turbocharger units is a costly operation, it has been the only option for remanufacturers wanting to update these old turbocharger units because of the lack of alternative viable methods for rebuilding a compressor housing while maintaining the strict positional and dimensional tolerances that are required for proper operation of the resultant remanufactured turbocharger.